1. Field Of The Invention
This invention relates to prosthetic joints generally, and more particularly to an improved, unconstrained prosthetic knee replacement for a dysfunctional knee.
2. Prior art
Referring now to prior art knee endoprostheses, there are basically two types of prosthetic replacement knees known generally as constrained and unconstrained knees. An example of an unconstrained or floating meniscal bearing knee is disclosed in Buechel et al Pat. No. 4,340,978. An embodiment of the Buechel invention is manufactured and sold by DePuy, Inc. of Warsaw, Indiana. Preferably, the bearing elements of these types of knees are manufactured with high density polyethylene such as that disclosed in Zachariades Pat. No. 4,587,163 developed by Polteco Inc. of Alameda, Calif. because of its superior wear resistant characteristics.
Referring next to typical prior art tibial-femoral knee prostheses, prostheses which allow axial rotation and A-P motion in addition to flexion-extension motion have incongruent contact (usually theoretical point-contact) between the femoral and tibial bearing surfaces, have been known. Those prior art knee prostheses which do provide congruent or area bearing contact fail to provide the needed axial rotation, or when cruciates are present the needed anterior-posterior motion.
Preexisting constrained knees have often resulted in early failure as a result of hinge constrainment. The degree of rotation was limited to either only one plane or a very small arc. Also, as shown in U.S. Pat. No. 4,219,893, very little flexibility was possible in the shape of the patello-femoral interfaces because of the requirement to maintain congruent patello-femoral contact over the range of motion of the knee. As a result, patello-femoral tracking problems became commonplace.
It was necessary to use a large circumference when used to resurface allografts resulting in problems with soft tissue necrosis and/or patello-femoral tracking problems as described above. Furthermore, most implants were known as custom devices since they had to be specially made to fit a particular patient's size and thus required excess manufacturing time and unnecessary delays.
An additional, significant problem with prior art constrained knees results from the fact that the range of motion prevents the normal A-P movement of the inferior end of the femur relative to the posterior end of the tibia. This "sliding" movement is necessary in order to maintain the full range of motion desired in a prosthetic device.
Current prostheses of the dislocatable cruciate retaining type, such as the Geomedic knee replacement shown in U.S. Pat. No. 3,728,742 to Averill et al, that produce area contact provide only one axis of rotation relative to the femur for the flexion-extension motion. Normal flexion-extension is, however, characterized by a polycentric flexion-extension motion where rotation relative to the femur occurs about many axes.
This polycentric motion, which results from the action of the cruciate ligaments and condylar shape, allows for more efficient utilization of muscle forces by providing a posterior shift of the axis when effective quadriceps action is important and an anterior shift when hamstrings effectiveness is important. Furthermore, in the human knee it is this action and the A-P shift, and the shape of the posterior condyles, which influence this motion so as to allow full flexion capability for the knee. Failure to provide appropriate knee geometry inhibits, when cruciate ligaments are present, this natural motion and thus tends to restrict muscle effectiveness and inhibit flexion. These restrictions tend to increase both loading on the prosthesis and loading between prosthesis and bone.
Another problem exists with regard to knee endoprostheses for implantation in those cases wherein the cruciate ligaments are functionally absent but where the collateral ligaments are functional or at least reconstructible. In the absence of cruciate ligaments, the prosthetic replacement must provide anterior-posterior knee joint stability so as to replace that stability otherwise provided by the cruciates. Until recently most such cases were treated by a constrained type knee prosthesis.
Where the cruciate ligaments are present, most surgeons would prefer their retention, since they provide important internal stabilizers and, together with the condylar geometry of the femur and tibia, control the rotation axis and A-P motion of the knee. Furthermore, these ligaments provide anterior-posterior stability. Thus, it is desirable to reserve the cruciate ligaments, even though reasonable stability can be provided by a properly designed full platform type prosthesis.
In addition, the action of the cruciate ligaments produces a shift in the rotation axis of the knee which results in more efficient muscle utilization. Thus, preservation of these structures provides better physiological function after knee replacement.
It is not, however, clear that the physiological advantages gained in retaining the cruciates outweigh the disadvantages of the design compromises, such as increased bearing surface incongruency and reduced tibial prosthesis bearing area, required to retain these ligaments. Thus, the desirability of retaining the cruciate ligaments in the cases of unconstrained knee replacement is not well established.
A recent unconstrained knee concept, the New Jersey knee, appears to provide a partial solution to the problem of overconstraint while attempting to maintain congruency by the use of meniscal floating elements. Unfortunately, this knee suffers from several design problems which appear to limit its usefulness.
An important consideration in the design of knee implant devices is maximizing performance in terms of providing ranges of motion commensurate with those of the natural knee being replaced. Another important consideration is coupling the above desired ranges of motion with suitable strength in each range of motion to accomplish normal body activities. The most difficult of normal body activities to restore and/or maintain following knee implant surgery include the ability to stand from a seated position and the ability to climb stairs. These activities tend to be most demanding in terms of the required range of motion and the level of forces applied to the various knee region components. Thus, the ability of a knee prosthesis to perform satisfactorily under these demanding conditions is one way to measure performance.
The natural human knee joint is a device having six degrees of freedom. It provides flexion/extension capability, varus/valgus ability and internal/external rotation ability relative to the longitudinal axes of the femur and tibia. In addition, the natural human knee provides anterior/posterior translation with the flexion/extension motion that is described above. The inventors believe that providing at least the same degrees of freedom as the natural knee in a knee prosthesis is most likely to provide a satisfactorily performing prosthesis.
Existing knee implant devices are directed toward two somewhat different types of resulting functions: (1) creating function in a replacement knee joint to approximate, as nearly as possible, the natural function of the original knee joint and (2) creating new function in a replacement knee joint that is different from the natural function of the original knee joint, but which is nevertheless suitable for accomplishing the desired normal body activities. The goals of achieving these types of functions can result in different approaches in the design of the implant devices themselves, the procedure used for implantation and the planned resulting interaction between the implanted devices and the remaining knee joint environment, including the remaining bone portions of the femur and tibia, ligaments, tendons and muscles. Some design considerations, on the other hand, are believed to be shared, at least in some aspects, between knee implants which attempt to approximate natural knee function and knee implants which create an entirely new type of function.
During flexion and extension of the natural knee, the femur shifts, or translates, in its position relative to the tibia, posteriorly and anteriorly, respectively. The posterior translations of the individual condyles of the tibia relative to the femur do not, however, occur over precisely the same distance during the flexion and extension activities. The translation of the lateral tibial condylar surface is actually greater than that of the medial tibial condylar surface for both flexion and extension. Accordingly, flexion and extension of the natural human knee are accompanied, respectively, by a component of internal and external rotation of the tibia relative to the longitudinal axis of the femur.
In addition, the natural center of this internal/external rotation of the tibia relative to the longitudinal axis of the femur tends to be slightly medial relative to the geometric center of the tibial plateau. Although this medial offset in the axis of internal/external rotation can be measured in different ways, it is believed to be best measured relative to anatomical landmarks of the knee, such as the tibial eminence, a pair of raised bone portions upon the superior tibial surface, called the medial and lateral tibial eminences, or the medial and lateral intercondylar tubercles. The natural medial offset of the internal/external rotation axis is believed to be aligned with the medial tibial eminence. Although the precise distance varies among individuals due to differing knee geometries and sizes, it is believed that the offset distance for the center of internal/external rotation relative to a point midway between the medial and lateral tibial eminences varies from approximately 5-10 mm. It is believed that for average-sized individuals, this offset distance will be approximately 7 mm.
Current knee prostheses are not designed to include this offset axis of internal/external rotation. Although the various force distributions, types of motions and the interactions of the various knee region components (including the femur, tibia, associated ligaments, tendons and muscles) among both natural and prosthetic knees are not completely understood, it is believed that consideration of the axis of internal/external rotation as an additional design feature is more likely to result in a knee prosthesis that more accurately mimics natural knee function. Consequently, inclusion of this additional design feature is believed to provide a performance improvement over existing designs.
It should also be noted that since the interactions of the remaining knee environment components mentioned above also contribute to knee function, changes made during the surgical procedure to these other components will also result in design changes, in some ways mimicking and in some ways compensatory, in final knee prosthesis construction. During surgeries for the implantation of knee prostheses, oftentimes one or more of the natural components of the knee region is either sacrificed or changed in its resultant geometry and/or connectivity. Such a change may result in different levels of reliance, at different strengths and/or in different directions, on the remaining natural components in order to achieve satisfactory function in the implanted knee.
In some implant procedures, for example, the anterior and posterior cruciate ligaments are removed. This change causes the motion of the implanted knee to be largely governed by the medial and lateral collateral ligaments. Because of the changes in geometry, rotational movement and force distribution between natural knee region components and the components of an implanted knee prosthesis, it is believed advantageous to incorporate additional design features into a knee prosthesis that adjust or compensate for these changes in ways that will be favorable for the motion and strength of the resulting joint.
An additional consideration in the design of knee prostheses involves maximizing the operational life of the prosthesis by minimizing wear of the prosthesis components. Extended wear resistance tends to require less revision/replacement surgery following the original implant. It also tends to allow more vigorous activity that is more demanding on the replacement joint, which is important for athletic-type activities and general exercise. Although wear considerations are pertinent to the design of all prosthetic knees, they currently tend to be especially pertinent to fixed bearing knees, as opposed to mobile bearing knees which include other features designed to minimize wear. Thus, improving the design of these implant devices is believed to best focus on the dual goals of minimizing wear with enhanced performance.
Many knee prostheses tend to wear most in the posterior-medial quadrant. The study of forces associated with movement of the natural human knee and knee implants has determined that anterior-posterior sliding, axial rotation and congruent versus incongruent bearing contact are important design considerations in a prosthetic knee for achieving favorable utility with minimum wear. Each of these considerations can affect both the degree and location of wear in prosthesis components over the ranges of knee motion, although the processes which contribute to the degree and location of wear are not fully understood. It is now believed, however, that other additional factors observed with respect to both natural and artificial knees are also influential toward achieving favorable utility with minimum wear. These additional factors include rotational forces distributed among individual knee condyles, the center of internal/external knee rotation between the femur and tibia at various portions of knee motion, the distribution of knee joint forces relative to the femur and tibia, and the individual lever arms associated with ligaments, tendons and muscles of the knee joint. In particular, it has been determined through the studies of both natural and prosthetic knees that these additional factors can be adjusted in various relative ways in a prosthetic knee, to enhance performance and minimize wear. Sometimes the adjustments made are designed to mimic natural knee region characteristics. Other times they are designed as a purposeful deviation from the natural condition to adjust relative to, compensate for, or take advantage of, changes resulting from the implant design and the resultant knee region characteristics following implantation. One factor believed to take several of these additional influential features into account is the center of internal/external rotation of the prosthesis relative to the longitudinal axes of the femur and tibia. Current prostheses are believed to have room for design improvements in this area that can enhance their function and extend their useful life.
The present invention, the Pottenger/Draganich Knee utilizes new concepts combined in an improved design in order to avoid some of the anticipated difficulties of the prior art design.